Issue 55

A. Gryguć et alii, Frattura ed Integrità Strutturale, 55 (2021) 213-227; DOI: 10.3221/IGF-ESIS.55.16

significant thermomechanical defects were present and their level of contribution on the nucleation, growth and final failure of the material. Shiozawa et al. investigated the dependency of stress amplitude level on the fracture surface morphology of extruded AZ80 Mg, and saw virtually no difference in the surface roughness in the area immediately surrounding the fatigue crack initiation (FCI) as well the fatigue crack propagation (FCP) zones for samples which were tested at both high and low stress amplitudes [34]. They also observed cracks which initiated at the surface with a singular origin on the sample, which is largely similar to those types of failures which are presented in this current study. The lack of dependence of the surface roughness surrounding the initiation zone on the nominal stress amplitude forms the basis of comparison for highlighting the presence of incipient defects near the FCI. More specifically, since the local stresses at the FCI will be amplified due to the stress concentration factor (SCF) superimposed by the incipient crack like defect, we can still compare the surface roughness to distinguish material which was fractured due to the applied fatigue load, from material which was poorly fused from a cold-shut type defect which behaves like an incipient crack. Fig. 4 illustrates the macroscopic morphology of the fracture surface and topological features with roughness assessment using quantitative LOM for a laboratory fatigue test specimen with geometry as per Gryguć et al [21]. This specimen was extracted from location #8 within the forged component which was free from these aforementioned macroscopic cold-shut/poor fusion defects, and then subsequently underwent fully reversed strain-controlled fatigue testing with a strain amplitude of ε A = 0.8% and failed at 568 cycles. The stabilized peak and valley stresses were +285 MPa and -230 MPa respecitvely. It can be observed in Fig. 4a that the FCI location is in the upper right-hand corner of the image, and that progresses diagonally across the sample cross section. Furthermore, the roughness observed in the direction parallel with the crack growth direction highlighted by the red line in Fig. 4a, is R a = 0.74µm which corresponds well with that which was observed by Shiozawa et al in the crack growth zones (yellow zone in Fig. 4b) in the fracture surface in AZ80 extrusion [34]. Fig. 4c highlights the roughness profile of the location denoted by the red line in Fig. 4a, large macroscopic fatigue striations can be seen which represent large unstable jumps in the crack growth at those locations within the FCP zone. This roughness profile can be thought of as a normalized surface profile to account for curvature or inclination of the surface in order to highlight the features of the surface topography on a more detailed level.

Figure 4: Fracture surface of strain-controlled fatigue test specimen extracted from location #8 of AZ80 Mg forged component (a) macroscopic view of fracture surface, (b) quantative LOM of fracture surface to highlitght macroscopic topological features (c) roughness profile of section A in the fatigue crack growth area of fracture surface with an average surface roughness of R a = 0.74.

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